libxfs: fix xfs_isset pointer calculation

libxfs: fix xfs_isset pointer calculation

In the macro xfs_isset, the variable 'a' is a pointer to an array
type.  However, the bit offset calculation uses sizeof(a), which
returns the size of the pointer, not the size of an array element.
Fix this, which also fixes the problem where xfs_check spits out
bogus "rtblock X expected type unknown, got rtfree" messages.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>